A light emitting diode, or LED, is a semiconductor device that emits a particular spectrum of incoherent light when a forward bias is applied across the LED's anode and cathode terminals. An LED is formed by doping a semiconductor material with various impurities to form a p-n junction that emits photons when current flows from the p-side of the junction (anode) to the n-side of the junction (cathode). The color, or wavelength, of light emitted by an LED depends upon the material that forms the diode's p-n junction. For example, an LED constructed from aluminum gallium arsenide (AlGaAs) radiates infrared and red light, one constructed from aluminum gallium phosphide (AlGaP) radiates green light, one constructed from gallium nitride (GaN) radiates green and blue light, one constructed from indium gallium nitride (InGaN) radiates near ultraviolet, bluish-green and blue light, etc.
Generally, LEDs are formed by depositing a p-type layer onto an n-type substrate. An anode pad, coupled to the p-type layer, is mounted to the top surface of the LED chip, and a cathode pad, coupled to the n-type substrate, is mounted to the top surface of the LED chip as well. LEDs may also be formed on transparent substrates, such as sapphire (Al2O3). For example, in a GaN-on-sapphire LED, at least one n-type layer is first formed on an upper surface of the sapphire substrate, and then one, or more, additional layers, including a p-type layer, is formed on the upper surface to create the p-n junction. Many well known processes may be used to form these layers, such as, for example, metal organic chemical vapor deposition (MOCVD), plasma deposition, etc. A reflective, metal layer is formed on the bottom surface of the sapphire substrate to reflect downwardly-emitted light back up through the top surface.
FIG. 1 depicts a prior art LED die 1 mounted to printed circuit board 10, i.e., a “chip on board” design. A p-n junction 2 is formed on the top surface of sapphire substrate 3, and a reflective, metal layer 4 is formed on the bottom surface of sapphire substrate 3. Wires 7, 8 are bonded to the anode and cathode pads 5, 6 (respectively) on the top surface of LED die 1, as well as to corresponding anode and cathode pads 11, 12 (respectively) on printed circuit board 10. Wire bonding, such as thermocompression, thermosonic, ultrasonic, etc., is the standard method by which wires 7, 8 are attached to LED pads 5, 11 and printed circuit board pads 6, 12 (respectively). Metal layer 4 reflects downwardly-emitted light from p-n junction 2 upward, ideally through the top surface of LED die 1. Consequently, while some of the light emitted by p-n junction 2 may escape through the sides of sapphire substrate 3, most of the light is emitted from the top surface of LED die 1. FIG. 2 presents a picture of top view of LED die 1 mounted to printed circuit board 10, showing wires 7, 8 bonded to LED pads 5, 6 (respectively).
LEDs have been used within in vivo, non-hermetically-sealed sensors, and, in these applications, printed circuit board 10 is commonly ceramic (alumina), or a ceramic composite, while LED substrate 3 is typically sapphire, silicon or another similar material. While these materials are generally impervious to water or water vapor, wires 7 and 8, LED pads 5 and 6 and printed circuit board pads 11 and 12 must be protected from the harsh environment of the human body. Consequently, these components are typically encased in a polymer material, which, unfortunately is prone to water or water vapor infiltration. Over time, this undesired water permeability not only affects the properties of the polymer but also promotes premature failure of the LED by various mechanisms, including, for example, dielectric constant degradation, oxidation, electrical shorts, void space formation, delamination of gold pads on substrates, etc.
FIG. 3 depicts the effect of water infiltration on a prior art LED die, mounted to a printed circuit board, whose electric connections have been encased in a polymer material. While printed circuit board 10 effectively blocks water from infiltrating into the electrical connections to LED die 1 from the bottom, water may ingress into the polymer material 15 from the remaining directions, as shown in FIG. 3. To emphasize the water permeability problem, polymer material 15 has been exaggerated in size in FIG. 3.
While it is known that LEDs can be mounted to printed circuit boards in an inverted manner, i.e., a “flip chip” orientation, these prior art techniques, by themselves, fail to overcome the problem of water permeability when LEDs are deployed in a harsh environment. Moreover, when compared to the standard, chip on board design, flip chip LEDs emit less light because light that is emitted through the lower surface of the LED, i.e., the surface that is closest to the printed circuit board, is generally scattered, i.e., not reflected back into the LED and out through the upper surface. Consequently, a prior art flip chip LED not only fails to address the problem of water-permeability within an in vivo, non-hermetically-sealed sensor, but also emits less light than a standard, chip on board design.